The present invention relates to a method of fitting a tire for an off road motorcycle, and a tire for an off road motorcycle, in which off road traction is improved while shock absorption is maintained.
Absorption of shock caused by uneven road surfaces, as well as so-called off road traction, has a great influence on the performance of a tire used for an off road motorcycle. For example, in order to keep the motorcycle going straight on an uneven road surface, tires having the following combination of specifications are used for an off road motorcycle of 250 cc displacement.
Size: 21 inches for a front wheel/19 to 18 inches for a rear wheel (a difference of greater than or equal to 2 inches is provided between the front wheel and the rear wheel)
Construction: a tire having a bias carcass for both the front and rear wheels
Aspect ratio: 100% for the front wheel/90 to 100% for the rear wheel
In order to further improve off road traction, it is effective to decrease the aspect ratio of the rear wheel tire so that the traction loss at the side portions of the tire is reduced. However, due to the construction of the tire, the rigidity of the tread portion is increased as the aspect ratio of the tire is decreased, thereby leading to considerable deterioration in so-called off road cushion characteristics (shock absorption). In addition, cushion characteristics are further deteriorated by this decrease in the aspect ratio (due to a reduction in thickness of the tire side portions), and therefore, it has been considered difficult to set the aspect ratio below the current aspect ratio, i.e., 100 to 90%.
In view of the aforementioned, an object of the present invention is to provide a method of fitting a tire for an off road motorcycle and a tire for an off road motorcycle, in which off road traction can further be improved while shock absorption is maintained, without impairing the basic characteristics of a current motorcycle.
A first aspect of the present invention is a method of fitting a tire for an off road motorcycle, wherein: the diameter of the rim of a rear wheel is one inch smaller than the diameter of the rim of a front wheel; and at least a tire of the rear wheel is substantially a tire having a radial carcass with an aspect ratio of less than or equal to 80% and has at least one spiral belt. By providing a one-inch difference between the diameter of the rim of the rear wheel and the diameter of the rim of the front wheel, the basic characteristics of the motorcycle can be maintained with the outer diameter of the rear wheel being substantially the same as that of a conventional rear wheel, and the aspect ratio of the rear wheel tire can further be reduced to 80 to 70%. Further, a so-called spiral belt is combined in conjunction with the radial construction employed for the rear wheel tire so as to moderately suppress the rigidity of the tread portion. Accordingly, it is possible to achieve both an improvement in the off road traction due to the configuration and the construction of the tires, and the ensuring of the cushion characteristics due to the radial carcass and the spiral belt.
A second aspect of the present invention is a tire for an off road motorcycle, wherein the tire is substantially a tire having a radial carcass with an aspect ratio of less than or equal to 80% and has at least one spiral belt. Since the tire for an off road motorcycle employs a spiral belt, it is possible to moderately suppress the rigidity of the tread portion as compared with a tire which employs a cross belt, and thus it is possible to maintain or improve cushion characteristics while maintaining traction performance.